Fast-start locomotor behavior is important for escaping from predators and for capturing prey. To examine the effects of size and other aspects of developmental morphology on fast-start performance, the kinematics of the fast-start escape behavior were studied through early post-hatching development in three salmonid species: chinook salmon (Oncorhynchus tshawytscha), coho salmon (Oncorhynchus kisutch) and brown trout (Salmo trutta). These three species, while morphologically and developmentally similar, hatch and mature at different sizes (total length). Comparison of these species shows that some fast-start performance variables, including stage duration, maximum velocity and maximum acceleration, are highly dependent on ontogenetic state, while another, the overall distance traveled during stage 2, scales with total body length. Brown trout were studied from hatching into the juvenile development period. Aspects of fast-start performance peak at the end of yolk-sac absorption (the end of the eleutheroembryo phase) when the fish reaches the juvenile period. At this time, the durations of the fast-start stages are at their minima, and maximum velocity and maximum acceleration are at their highest levels relative to body length. Thus, escape behavior reaches its maximum size-specific performance at a relatively small size, just as the fish absorbs its yolk sac and begins to search for food. This peak in fast-start performance occurs during a life history period in which fast-start ability is likely to be particularly important for survival.